a. Field of the Invention
Broadly speaking, this invention relates to optical fibers. More particularly, in a preferred embodiment, this invention relates to methods and apparatus for manufacturing an optical fiber which is relatively insensitive to the effects of stress and bending.
B. Discussion of the Prior Art
As is well known, the feasibility of transmitting communication signals over optical fibers has been successfully demonstrated in the laboratory. Attention is currently focused on the development of practical systems for use in the field. The present thinking is that one of the first commercial uses of such optical communication systems will be in densely populated metropolitan areas, such as New York City, where existing cable ducts are straining at the seams and the cost of construction new duct work is prohibitively expensive. Under these circumstances, optical cables are extremely attractive because of their inherently smaller cross-section.
There are, of course, significant differences between the copper conductors used in conventional telecommunications systems and the fibers used in optical communications systems. A conventional cable comprising a plurality of insulated copper conductors, for example, may be subjected to considerable tension and to severe bending without significantly affecting transmission through the cable. Optical fibers, on the other hand, being comprised of glass are considerably more delicate and when subjected to excessive tension the fibers break. When subjected to even moderate bending of the right periodicity, the coupling between guided and unguided modes in the fiber will dramatically increase, resulting in unacceptably high radiation losses; hence, severe attenuation of the signals carried by the fiber.
It is known that by encapsulating optical fibers in protective sheaths of soft and hard plastics, the radiation problem can be solved but the thickness of these plastics may make the cable too large for their intended use.